Styrene-butadiene copolymer resins (hereinafter, also referred to as “SBC resin”) have high flexibility due to a butadiene rubber component contained therein and in addition have excellent transparency and processability, and by virtue of the features, they have been conventionally used for a wide variety of applications such as packaging materials for food, packaging materials for electronic parts, blister packages, and toys.
Although an SBC resin may be used singly in these applications, it is more common that an SBC resin is used in a blend with a styrene homopolymer resin (GPPS), a polystyrene resin such as an high impact polystyrene (HIPS), or a styrene-alkyl (meth)acrylate copolymer resin. The reason is, for example, that such blending facilitates control of balance between impact resistance and stiffness and balance between mechanical characteristics and economic efficiency in the material design. Specifically, it is only required to prepare two starting materials of an SBC resin and a polystyrene resin and adjust the blending ratio between them to better meet the properties of a product, and moreover a wide variety of materials can be produced in accordance with an object.
For example, Patent Literature 1 discloses a composition containing an SBC resin and a polystyrene resin.
There exist various methods for molding a thermoplastic resin, and typical representatives thereof are injection molding and extrusion molding. Molding machines for them generally include a screw. Functions required for the screw include “transporting (conveying)”, “melting (fusing)”, and “kneading (dispersing)” a thermoplastic resin. In particular, a T-die extruder in extrusion molding and a twin-screw extruder for the purpose of kneading (compounding), and the like use a screw designed to be capable of “transporting (conveying)”, “melting (fusing)”, and “kneading (dispersing)” in accordance with a thermoplastic resin to be used and the object.
In contrast, a general-purpose screw designed to have a function primarily for melting/plasticizing of a thermoplastic resin is used for a screw for an injection molding machine, and the screw in fact only has abilities to “transport (convey)” and “melt (fuse)”, without an ability to “knead (disperse)” in many cases.
However, the number of applications involving improvement of physical properties, such as polymer blending, is increasing in association with the recent diversification of applications of resin products, and cost reduction with use of an injection molding machine in combination with maintaining high quality for a resin product is increasingly required year by year also for thermoplastic resins.
Thus, when an injection-molded product is produced from a composition containing an SBC resin, a styrene resin, and the like, a pellet of a blended composition of an SBC resin, a styrene resin, and the like, is temporally produced by using extrusion molding, and thereafter injection molding is carried out to fabricate a resin product, as a result of which two molding steps which have an influence on thermal history are required. On the other hand, direct fabrication of a resin product by using injection molding of a dry blend of an SBC resin and a styrene resin, without the step of producing a pellet of a blended composition of an SBC resin and a styrene resin by using extrusion molding, requires only one molding step, for example. As a result, improvement in quality from the viewpoint of suppression of the deterioration of a resin and reduction of a burnt resin through the reduction of thermal history can be achieved in combination with cost reduction through reduced production steps.